1. Field of the Invention
The present invention generally relates to a speed change control apparatus for an engine. More particularly, the present invention relates to techniques for achieving a speed change, during an output shaft of a transmission rotates at a predetermined speed, without using a clutch or the like.
2. Description of the Related Art
Heretofore, various kinds of systems, for example, a system having a clutch provided between an internal combustion engine and a motor for running, and a system having a transmission provided between an internal combustion engine and a motor for running or between a motor for running and a drive wheel, have been proposed as composite engine systems for a vehicle, which are adapted to run depending upon motive power provided by a motor for running (a main motor) as much as possible when waiting for a traffic signal light to change or during moving slowly and adapted to run depending upon motive power provided by a heat engine such as an internal combustion engine when being accelerated or during moving at somewhat high speed, so that both ultimate energy-saving effects and realizing low-pollution vehicles can be realized at the same time (see, for instance, Japanese Unexamined Patent Publication (Kokai) Nos. 5-59973, 7-170610, 8-266012, 7-277009, 7-277014 and 7-304343).
Further, Japanese Patent Application No. 8-196557 discloses another system similar to the aforementioned systems in concept. It has been known that the system disclosed in this Japanese Patent Application has special effects by being configured in such a manner as to comprise a second transmission as described above. Incidentally, the term "transmission" referred to herein does not designate a reduction gear adapted to realize a fixed speed change ratio, but means a mechanism capable of realizing at least two speed ratios.
Meanwhile, theoretically, such conventional engines and control systems having clutches and transmissions are considered as ultimate systems. However, when put to practical use, such conventional engines and control systems have the following drawbacks:
First, in consideration of the size of the transmission, a limit is imposed on the transmission capacity thereof. Especially, it is difficult to apply a continuously variable transmission to a truck. PA1 Second, the continuously variable transmission is more efficient than a torque converter. However, the transmission efficiency of the continuously variable transmission is less than that of a gearbox. PA1 Third, in view of the rather high transmission capacity of the transmission, a strong control section is needed. PA1 Fourth, it is difficult to realize a low-cost and high-capacity system while ensuring the reliability and durability thereof.
On the other hand, in a somewhat high speed running area where a driving force depends upon an output of the internal combustion engine, the frequency of repetition of acceleration and deceleration is low, differently from the case of running in a city where the repetition of acceleration and deceleration is frequently performed. It is, thus, observed that the speed of actual traffic is nearly constant in such an area. Further, the conventional system is equipped with a continuously variable transmission for the purpose of ensuring a driving force in a very low speed running area. Such a purpose has been relatively easily accomplished by providing a simple transmission between the motor and the wheel.
It is very desirable for realizing a low-pollution internal combustion engine and energy-saving that the operating speed of the internal combustion engine is maintained at a constant value independent of a change in vehicle speed, as previously proposed. To achieve this, the system should be necessarily provided with a continuously variable transmission. This continuously variable transmission is a composing element having an advantage in continuously changing a rotation speed but is a mechanism including a large stress therein. If the aforementioned condition is moderated by replacing the term "at a constant value" with the expression "within a certain range", the system may employ a mechanical transmission that uses ordinary gears.
This system retrogresses from the conventional system that features the ability to operate the internal combustion engine at exactly constant speed and torque. However, if the internal combustion engine is permitted to change the operating speed thereof within a certain range according to a vehicle speed, a system capable of achieving effects similar to those obtained by the functions of the conventional system can be realized by providing a mechanical transmission therein.
Incidentally, in a method of operating the engine, attention should be paid to the following points in a low-speed running area, during acceleration, and in medium-speed and high-speed cruising areas, as compared with a method of operating the conventional composite engine system constituted by using a continuously variable transmission.
1. In a low-speed running area, an operation of the internal combustion engine should be stopped. Alternatively, the system should be adapted to function as a serial hybrid one.
Namely, in the low-speed running area, there is a large difference between the power provided in an essentially desirable operating region of the internal combustion engine and the power needed by a motor vehicle (namely, excessive power of the internal combustion engine). During a cooperative operation of the internal combustion engine and the motor, this motor functions as a large generator at all times. Especially, during deceleration, the kinetic energy of the vehicle body is regenerated. It is, thus, difficult to efficiently store such a large current. Moreover, in this area, vehicles frequently repeat short-time acceleration and deceleration. Thus, operations of controlling the engine system are complicated and troublesome. Furthermore, there is a fear that the durability of the system is impaired. Particularly, regarding the problem of torque during deceleration, when the rotation speed at an axle position is reduced by the continuously variable transmission in accordance with the vehicle speed while performing the constant operation of the internal combustion engine at a constant output, a large driving force is produced at the axle despite an intention of deceleration. Th is results in emergence of a situation where the motor overworks on a braking operation in the low-speed running area in which the electric-power generating ability thereof drops off and where the system is under stress. It is advisable that the generation of motive power for running is left to the motor and that the internal combustion engine is caused to stop or to devotedly generate electric power within a relatively small output range.
With such a novel idea, the internal combustion engine is separated from the generation of motive power in the low-speed running area where the speed change of the vehicle frequently occurs. Moreover, the necessity of the continuously variable transmission is reduced. Furthermore, the possibility of employment of the mechanical transmission is increased.
2. During acceleration, the driving force allotted share depending upon the internal combustion engine i s somewhat shifted to the driving force allotted share depending upon the motor.
Namely, in the case of the vehicle having the conventional system, an accelerating force is short if only the driving force generated by the motor is used in the low-speed running area. Thus, such a conventional vehicle is inferior to the current motor vehicle in performance. Consequently, such a conventional vehicle employs a method of first reducing the driving force of the internal combustion engine by a continuously variable transmission or the like, and subsequently obtaining a large driving force in the low-speed running area and further superimposing the obtained driving force upon a driving force generated by the motor and then transmitting the super imposed driving forces to a drive shaft. However, in environments where mechanical transmissions are employed instead of continuously variable transmissions, a steep change in the engine rotation speed of the internal combustion engine in a very short time is liable to frequently take place in the low-speed running area. As described above, outputs of the internal combustion engine are not available during acceleration if the rotation of the internal combustion engine is disconnected from or is indirectly connected to the drive shaft. Thus, the conventional system should take another means for supplementing an accelerating force. It is, therefore, considered that a mechanism, which has a transmission provided between the motor and the axle and is operative to increase the torque generated by the motor during acceleration or up-hill climbing and to then transmit the torque to the axle, is suitable for such a means. Namely, the concept, in which, by reducing the rotation speed, the rotating force of the internal combustion engine is increased to be utilized, in the situation where a large driving force is necessary, for example, on acceleration, is applied to a case where the torque generated by the motor is utilized by being increased, during acceleration accompanying vehicle speed change, while using outputs of the internal combustion engine when a large driving force of a constant speed is necessary, for instance, during up-hill climbing.
3. In the medium-speed and high-speed cruising areas, the driving force allotted share depending upon the internal combustion engine is increased.
Namely, vehicles of the conventional structure have a problem in that such vehicles are not in the environment where outputs of the internal combustion engine are efficiently utilized in the medium-speed and low-speed running areas. However, the internal combustion engine often achieves the performance thereof most satisfactorily in the medium-speed and high-speed cruising areas, especially, in the high-speed cruising area. It is known from experience that the more needless operations are performed in the high-speed cruising area, the more energy-saving effects are lost. In this high-speed cruising area, it is very improper to use what is called a serial hybrid vehicle adapted to run by first generating electric power by use of the rotating force of the internal combustion engine and then converting the generated power into the rotating force by the motor. The use of the serial hybrid vehicle in the high-speed cruising area results in exhaustion of the gain obtained in the low-speed running area. In the high-speed running area, it is very important to transmit outputs of the internal comsbution engine to the axle as directly as possible. Comprehensive effects are easily obtained by limiting the functions of the motor to auxiliary ones such as the assistance during acceleration and the regeneration of power during deceleration.
Namely, it is more desirable for saving energy and preventing pollution that a tendency to use the motor as a main engine is strengthened under ordinary running conditions and that the degree of dependence of motive power upon outputs of the internal combustion engine is increased in a highway driving mode.
It is conjectured that a system being close or equivalent to the system having a continuously variable transmission may be realized by transmitting the rotating force of the internal combustion engine to the wheels without energy conversion and increasing the degree of dependence of motive power upon the motor in the low-speed running area while maintaining high efficient running in a high-speed cruising mode. A very compact well-organized composite engine system may be manufactured at low cost by employing a mechanical transmission. Moreover, a transmission for use in a large truck may be manufactured according to accumulated conventional technical knowledge by employing field-proven gears. This enables the application of the composite engine system to all motor vehicles.
Attention should be paid to these respects similarly as in the conventional case. However, in the case of the composite engine system employing a mechanical transmission, the aforementioned respects should be slightly emphasized, as compared with the composite engine system constituted by using a continuously variable transmission.
The present invention is accomplished in view of the problems of the conventional systems. Accordingly, an object of the present invention is to provide a speed change control apparatus for an engine system that employs a mechanical transmission using ordinary gears.